Precharging load capacitor for power-factor-corrected AC-to-DC power supply

ABSTRACT

A power supply for a capacitive-resistive load includes plural paralleled phase correcting modules together with current sharing controllers for tending to equalize their currents. Each module is provided with a diode, poled to prevent forward current from flowing in the return current path, for aiding in equalizing module currents. Surge currents are reduced by a single saturable reactor coupled to the combined outputs of current sharing controllers, thereby avoiding the need for soft-start in each controller. A precharging path extends from a source of pulsating direct voltage to the load, for precharging the load capacitance at turn-on.

GOVERNMENTAL INTEREST

This invention was prepared under government contract N00014-99-2-0002(HBMRS). The United States Government has a non-exclusive,non-transferable, paid-up license in this invention.

FIELD OF THE INVENTION

This invention relates to electrical power supplies, and moreparticularly to paralleled power supplies in which, variously, turn-onsurge currents are controlled, unidirectional return currentequalization is assured, and a capacitive load is precharged.

BACKGROUND OF THE INVENTION

It is often necessary to parallel power supplies in order to achieve adesired level of power. Such paralleling allows the use of standardizedor commercial-off-the-shelf (COTS) modules or units to achieve a levelof power which might otherwise require a costly custom-designed powersupply. For example, most single-phase power-factor-corrected (PFC)boost AC-DC power supplies available as COTS modules offer no more than1 KW of power capacity, and must be paralleled in order to provide, say,10 KW. FIG. 1 is a simplified diagram in block and schematic formillustrating a prior-art paralleled power supply 10 for providing directvoltage to a capacitive load 12. In FIG. 1, the capacitive load 12includes a load resistor 14 which represents the real orenergy-absorbing portion of the load, and a paralleled capacitor 16which represents the quadrature or out-of-phase (imaginary) portion ofthe load. The capacitor 16 may be an actual discrete capacitor orcapacitor bank, and it may also include the stray capacitance of variouscomponents and or connections. One end of resistor 14 and capacitor 16is connected to a load reference or ground conductor LG, and the otherends are connected to a load hot terminal LH.

In FIG. 1, a source of alternating current, such as power-line mains, isillustrated as 18. The source of alternating voltage drives a full-waverectifier represented as a block 20, which as known produces pulsatingdirect voltage (also known as pulsating direct current) represented by asymbol 22. Pulsating direct voltage is characterized by unidirectionalhalf-sinusoids of voltage, with the voltage value between voltage peaksgoing to approximately zero volts. The pulsating direct voltage may beviewed as being established or generated between a first commonconductor 24 relative to a common second or reference conductor 26. InFIG. 1, a plurality 28 of standardized single-phase switching phasecorrecting power-supply boost modules 28 a, 28 b, . . . , 28 n areconnected to conductors 24 and 26 for receiving pulsating direct voltagefrom rectifier 20, and for generating direct voltage for ultimateapplication to the load 12. Each power supply module of set 28 includesfirst and second power input terminals. More particularly, power supply28 a includes first and second power input terminals or ports 28 ai 1and 28 ai 2, respectively, which are connected to common powerconductors 24 and 26, respectively. Similarly, power supply 28 bincludes first and second power input terminals or ports 28 bi 1 and 28bi 2, respectively, which are connected to common power conductors 24and 26, respectively, and power supply 28 n includes first and secondpower input ports 28 ni 1 and 28 ni 2, respectively, which are connectedto common power conductors 24 and 26, respectively. It should be notedthat the term “port” formally includes a pair of terminals orelectrodes, but common usage extends the definition. Each power supplyof set 28 also includes first and second power output terminals, andmore particularly power supply 28 a includes first and second outputterminals 28 ao 1 and 28 ao 2, power supply 28 b includes first andsecond output terminals 28 bo 1 and 28 bo 2, and power supply 28 nincludes first and second output terminals 28 no 1 and 28 no 2. Oneexample of such single-phase power-factor correcting boost power supplymodules is model PFC-1000 manufactured by RO Associates, Inc. of 246Caspian Drive, P.O. Box 61419, Sunnyvale, Calif. 94088.

Each switching power supply module or element of set 28 of powersupplies of FIG. 1 includes internal circuitry, which may or may not beknown to the user. Such power supplies almost always include an inputinductor, which is represented in FIG. 1 by inductors 28 aI, 28 bI, . .. , 28 nI connected to the first input ports 28 ai 1, 28 bi 1, . . . ,28 ni 1 of power supplies 28 a, 28 b, . . . , 28 n, respectively. Thepower supplies also often include a unidirectional current conductingdevice, illustrated as a diode or rectifier 28 aD, 28 bD, . . . , 28 nD,through which an output or integrating capacitor is charged. In powersupply 28 a of FIG. 1, these capacitors are represented by a capacitordesignated 28 aC, and capacitors 28 bC and 28 nC of power supplies 28 band 28 n correspond. The integrating capacitor 28 aC, 28 bC, . . . , 28nC of each of the power supply modules 28 a, 28 b, . . . , 28 n isconnected across the output terminals 28 ao 1, 28 ao 2; 28 bo 1, 28 bo2; . . . ; 28 no 1, 28 no 2 of the module, for providing a low outputimpedance. Each switching power supply of set 28 also includes a currentsensing resistor for sensing the current flow in the return path. InFIG. 1, power supply 28 a has a return current sensing resistor 28 aR,power supply 28 b has a return current sensing resistor 28 bR, and powersupply 28 n has a return current sensing resistor 28 nR. The purpose ofthese return current sensing resistors in the various switching powersupply module or element of set 28 is to provide a signal representingthe return current at the second input terminal; this return currentsignal is compared by a comparator (not illustrated) with a scaledversion of the full-wave rectified voltage 22 to produce an errorsignal, which error signal forces the return current to follow or trackthe full-wave voltage, thereby forcing the current to be in-phase withthe applied voltage, which is the essence of phase correction. Eachpower supply 28 a, 28 b, . . . , 28 n of set 28 is also associated witha further return current equalizing resistor R1, R2, . . . , Rn of a set29 of return current equalizing resistors. More particularly, each powersupply 28 a, 28 b, . . . , 28 n of set 28 is also associated with afurther return current equalizing resistor R1, R2, . . . , Rn,respectively, which is connected between the return current outputterminal and the load ground LG. Thus, resistor R1 is connected toreturn current output terminal 28 ao 2 of power supply 28 a and to LG,resistor R2 is connected to return current output terminal 28 bo 2 ofpower supply 28 b and to LG, and resistor Rn is connected to LG and tothe return current output terminal 28 no 2 of power supply 28 n.

Within each switching power supply module or element of set 28 of powersupplies of FIG. 1, a “line current shaping controller LCSC andassociated power FET perform the boost power conversion. When the FET ofa module of set 28 is ON or conducting, energy is stored in theassociated input inductor (28 aI, 28 bI, . . . , 28 nI) associated withthe input port of the module. When the FET goes OFF or becomesnonconductive, the inductor produces a reaction voltage which adds tothe input voltage to produce the boosted output voltage. At the sametime, the average input port current follows the shape of the full-waverectified or pulsating direct input voltage 22.

In theory, it should be possible to simply connect the output terminalsof the various power supplies of FIG. 1 to the load 12. However, someproblems arise when the power supplies are paralleled in this manner andconnected to the load. A first problem is that the internal impedancesof the various power supplies 28 a, 28 b, 28 n may not be equal, withthe result that the current provided by each module may differ from thecurrent provided by the other modules. Such differences in internalimpedance may be the result of differences in the gain of the feedbackcircuits, which as known tends to change the impedance. It may alsoarise as a result of stray differences in connection resistances. Suchcurrent-sharing problems are controlled in the prior art by a set 30 offorward current sharing controllers, including current-sharingcontrollers 30 a, 30 b, . . . , 30 n, which tend to maintain the sameforward current to the load from each power supply module of set 28.Current-sharing controller 30 a has an input port 30 ai connected tooutput terminal 28 ao 1 of power supply module 28 a and an outputterminal 30 ao connected to load conductor LH, and further includes aconnection 30 ar to ground conductor LG. Current-sharing controller 30 bhas an input port 30 bi connected to output terminal 28 bo 1 of powersupply module 28 b, an output terminal 30 bo, which is connected to loadconductor LH, and a reference terminal 30 br, which is connected toground conductor LG. Current-sharing controller 30 n has an input port30 ni connected to output terminal 28 no 1 of power supply module 28 n,an output terminal 30 no connected to load conductor LH, and a referenceterminal 30 nr connected to ground conductor LG. Thus, the output portsof the current sharing controllers of set 30 are connected in common toload supply conductor LH. Each of the current sharing controllers of set30 is also connected by a reference terminal to ground conductor LG. Thecurrent sharing controllers of set 30 are of the soft ramp-up variety,to thereby prevent surge currents from occurring when the initiallyuncharged load capacitor 16 is connected to the charged output capacitor28 aC, 28 bC, . . . , 28 nC of any one of the power supply modules ofset 28. Such surge currents, as known, may be large enough to causefailure of a capacitor or the interconnections, or to reduce their lifeexpectancy. FIG. 7 is a simplified diagram in schematic form of aprior-art current sharing controller 30 with soft start.

FIG. 7 is a simplified schematic diagram of a prior-art soft-startcurrent sharing controller, together with some ancillary circuits. Fordefiniteness, the controller of FIG. 7 is designated as 30 a. In FIG. 7,current sharing controller 30 a includes a power FET (PFET) having itspower current controlling path connected to input terminal or port 30 aiand, by way of a series current sensing resistor 710, to output terminalor port 30 ao. Output port 30 ao of current sharing controller 30 a isconnected by way of a terminal 724 a to a common node 726. Other currentsharing controllers (not illustrated in FIG. 7) are connected to commonnode 726 by way of terminals 724 b, . . . , 724 n. A current sensor 728senses the total current supplied by all the current sharingcontrollers, and generates a current sense signal on a path 730. Path730 carries information about the total current to a current share inputterminal 732 a.

In FIG. 7, the gate of the PFET is connected to input port 30 ai by wayof a resistor 712, which provides the PFET with gate voltage morepositive than the voltage at output port 30 ao to tend to hold the PFETconductive or ON. The gate of the PFET is also coupled to the collectorof an NPN bipolar transistor 714. The emitter of transistor 714 isconnected to ground by way of an emitter resistor 716. When transistor714 is ON, collector current flows through resistor 712, and turns OFFthe PFET by reducing its gate current toward zero volts. The base oftransistor 714 is driven by way of a resistor 718 from the output of acomparator (a high-gain amplifier) 720. When comparator 720 tends tohigher output, transistor 714 conducts more and the PFET conducts less.A current regulating arrangement includes resistor 710 and a bipolar PNPtransistor 722. When the output current of current sharing controller 30a becomes large enough, the base-emitter junction of transistor 722becomes forward biased, and the transistor becomes active. When active,transistor 722 adjusts the voltage at the positive (+) input terminal ofcomparator 720, to tend to drive its output positive and thereby turnOFF the PFET. The inverting (−) input terminal of comparator 720 isconnected to A “startup” signal is generated by an external logiccircuit (not illustrated) which uses a variety of logic schemes todetermine the existence of a start-up condition, and a start-up signalis applied to the noninverting input terminal of comparator 720 by wayof an intermediary FET 736.

Improved paralleled power supply arrangements are desired.

SUMMARY OF THE INVENTION

An electrical apparatus according to an aspect of the invention is forpowering a load including a resistive and a capacitive component. Theelectrical apparatus comprises a source of pulsating direct voltage, anda first plurality of power factor correction units coupled to the sourceof direct voltage, each for converting the pulsating direct voltage intoa direct voltage at an output port, and for tending to maintain thecurrent through the source of pulsating direct voltage in-phase with thepulsating direct voltage. The apparatus also includes a plurality, equalto the first plurality, of current sharing controllers, each of whichcurrent sharing controllers includes a port coupled to the output portof one of the power factor correction units, and each also includes anoutput port in common with all output ports of the current sharingcontrollers. The current sharing controllers are subject to surgecurrent when the direct voltage of the associated one of the powerfactor correction units is coupled to the capacitive component of theload at turn-on. The apparatus also includes controllable switch meanscoupled to the source of pulsating direct voltage and to the load, forprecharging the capacitive component of the load.

In one version of this aspect, the power factor correction units areboost power factor correction converters. A desirable version furthercomprises a plurality, equal to the first plurality, of ground currentequalizing impedances coupled between a common reference terminal and acurrent return port of each of the power factor correction units; theseequalizing impedances preferably include unidirectional currentconduction characteristics, as in the case of a diode. Another desirableversion further includes means, such as a saturable reactor, fordelaying application of the direct voltage to the load.

In a preferred embodiment of the invention, the controllable switchcomprises unidirectional current conducting means, and theunidirectional current conducting means is poled to conduct when thepulsating direct voltage exceeds the load voltage. In this preferredembodiment, when the power factor correction units are boost converterswhich generate direct voltage having a first value greater than the peakvalue of the pulsating direct voltage, the unidirectional currentconducting means becomes nonconductive when the pulsating direct voltagedrops below the first value.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified diagram of a prior-art paralleled power supply,showing unwanted paths along which forward current may flow;

FIG. 2 is a simplified diagram of a paralleled power supply according toan aspect of the invention, in which unidirectional current conductingdevices are placed so as to prevent flow of forward current among theparalleled units, and showing paths by which current flows;

FIG. 3 is a simplified diagram similar to that of FIG. 1, showing asaturable reactor which tends to suppress surge currents at turn-on;

FIG. 4 is a simplified magnetization curve of a saturable material;

FIG. 5 is a simplified diagram similar to that of FIG. 1, showing theuse of a controlled current path for precharging a load capacitance atturn-on in accordance with an aspect of the invention;

FIG. 6 is a simplified diagram illustrating a combination of thearrangements of FIGS. 2, 3, and 5;

FIG. 7 is a simplified diagram in schematic form illustrating aprior-art soft-start current sharing controller which may be used in thearrangements of FIGS. 1, 2, and 5; and

FIG. 8 is a simplified diagram in schematic form illustrating a currentsharing controller arrangement which may be used in conjunction with thesaturable reactor embodiment of FIG. 3.

DESCRIPTION OF THE INVENTION

It has been discovered that the arrangement of FIG. 1 may not be asstable or consistent in performance as desired. More particularly, ithas been discovered that a forward cross circulation current,represented in FIG. 1 as a dash line 40, can flow from one PFC module toanother, as for example from PFC module 28 a to PFC module 28 b,returning to conductor 26. This cross circulation current tends todisrupt the current sensing mechanism of the affected module, andeventually the AC line current shaping. In addition, the uncontrolledcirculation may easily exceed the rating of the current-balancingresistors of the PFC modules, such as resistor 28 bR of module 28 b, forexample, and lead to component destruction. Further, the crosscirculation current also causes signal ground drift (reference shift)and erroneous signal processing.

Circulation of cross currents from one module to the others is preventedby the use of unidirectional current conducting devices such asrectifiers or diodes (diode). In FIG. 2, a diode or rectifier of a set210 of unidirectional current conducting devices is connected in serieswith a return current equalizing resistor of set 29. More particularly,a diode 210 a is connected in series with resistor R1, a diode 210 b isconnected in series with resistor R2, and a diode 210 n is connected inseries with resistor Rn. The diodes of set 210 are poled to allow theflow of return current to the module in question, but prevent the flowof forward current from the second output port of each power-supplymodule. More particularly, diode 210 a is poled with its cathodeadjacent second output port 28 ao of power supply module 28 a, diode 210b is poled with its cathode adjacent second output port 28 bo of powersupply module 28 b, and diode 210 n is poled with its cathode adjacentsecond output port 28 no of power supply module 28 bn. With the cathodesadjacent the output return current ports, forward current cannot flowfrom an output return current port, and therefore cannot flow into thereturn current port of another power supply module. Instead, the forwardcurrent in each power-supply module of set 28 flows in a path,illustrated in conjunction with power-supply module 28 a, extending fromconductor 24, through the first input port 28 ai 1 of the power-supplymodule, through at least the internal capacitor 28 aC, through theinternal current sensing resistor 28 aR, and out to conductor 26.

The cost of providing soft-start current ramp-up in each of thecurrent-sharing controllers of set 30 of FIG. 1 may be excessive. Theneed for soft-start current ramp-up in each current-sharing controlleris avoided by the addition of a single saturable reactor between theparalleled power supply modules and the load. More particularly,referring to FIG. 3, a saturable reactor 50 is connected in seriesbetween conductor portions LH′ and LH″, between load 12 and theparalleled output terminals 30 ao, 30 bo, . . . , 30 no of set 30 ofcurrent sharing controllers. A saturable reactor has a magnetic corewhich is characterized by a BH curve 55 such as that illustrated in FIG.4, where B is the magnetic induction and H is the magnetizing force. Theincremental induction, represented by the slope of curve 55, is maximumnear the center of the curve, and is much less at the ends of the curve.The regions of large slope represent operating regions in which theinductor has a large reactive impedance, and the zero-slope regions atthe ends of the curve represent regions in which the inductor has littleor no reactive impedance. The magnetic core of the saturable inductor isselected in conjunction with the number and layout of turns in order toprovide maximum induction and inductance at high rate of load currentchanges, and low or zero induction and inductance at low rate of currentof load resistor 14. The relatively large inductance presented by thesaturable inductor 50 of FIG. 3 to rapidly changing or surge currentstends to suppress surges. Thus, any of the phase correcting power-supplymodules or units of set 28 which may tend to produce a surge currentfinds that such a surge is opposed by a reaction of saturable reactor50. The opposition to the surge essentially suppresses the surge. Sincethe presence of saturable reactor 50 tends to suppress any surgecurrents flowing to the capacitive component 16 of load 12, the set 30of current sharing controllers need not have soft-start characteristics.In general, the use of a single saturable reactor, such as reactor 50,will be cheaper and more reliable than the use of a soft-startcontroller. FIG. 8 is a simplified diagram illustrating a currentsharing controller similar to that of FIG. 7, but in which thesoft-start feature is absent, and the signal paths required fordistributing startup signals to the various controllers are also absent.

Circuit arrangement 500 of FIG. 5 is similar to circuit arrangement 10of FIG. 1, and corresponding elements are designated by the samealphanumerics. Circuit arrangement 500 differs from circuit arrangement10, in accordance with an aspect of the invention, by the addition of aprecharging current path including a diode (D) 60. The prechargingcurrent path extends from conductor 24 at the output of full-wave bridgerectifier 20 to conductor LH adjacent the load 12. In operation atturn-on, the pulsating direct voltage 22 produced by rectifier 20 isimmediately applied to the anode of diode 60, and current flows throughdiode 60 and load capacitance 16, thereby charging capacitance 16 evenin the absence of significant voltage at the output terminals 28 ao 1,28 bo 1, . . . , 28 no 1 of the set 28 of power-factor correctingmodules. Thus, by the time the set 28 of power-factor correcting modulesreaches a nominal output voltage and the set 30 of current sharingcontrollers couples the set 28 of power-factor correcting modules toload 12 by way of conductor LH, the load capacitance 16 is already atleast partially charged. The precharge applied to load capacitance 16tends to reduce the magnitude of surge currents which might occur whenthe current sharing controllers couple the power-factor correctingmodules to the load.

It should be noted that if the power factor correction modules of set 28of FIG. 5 are voltage boost modules producing a direct output voltagewhich exceeds the peak value of the pulsating direct voltage 22 producedby rectifier 20, the precharging path including diode or rectifier 60will be turned OFF or become open-circuited, because the greaterpositive value of the direct voltage applied to the cathode of device 60by comparison with the lesser positive value of the pulsating directvoltage 22 will result in reverse bias of the diode or rectifier. Thisarrangement avoids the need for a separate switch and timing circuit todisconnect the precharging path.

FIG. 6 illustrates a circuit arrangement similar to that of FIG. 1, withthe inclusion of a set 210 of unidirectional current conducting devicesconnected in a manner similar to that described in conjunction with FIG.2, and also including a saturable reactor 50 as described in conjunctionwith FIGS. 3 and 4. In addition, the arrangement of FIG. 6 also includesa precharging device or path 60 corresponding to that of FIG. 5. Thesechanges to the arrangement of FIG. 1 tend to improve the performance ofthe parallel supply.

Thus, an electrical apparatus (500, 600) according to an aspect of theinvention is for powering a load (12) including a resistive (14) and acapacitive (16) component. The electrical apparatus (500, 600) comprisesa source (20) of pulsating direct voltage (22) and a first plurality (n)of power factor correction units (28 a, 28 b, . . . , 28 n) coupled tothe source (20) of direct voltage (22), for converting the pulsatingdirect voltage (22) into a direct voltage at an output port (28 ao 1, 28bo 1, . . . , 28 no 1), and for tending to maintain the current throughthe source of pulsating direct voltage in-phase with the pulsatingdirect voltage (22). The electrical apparatus (500, 600) also includes aplurality equal to the first plurality (n) of current sharingcontrollers (30 a, 30 b, . . . 30 n), each of which current sharingcontrollers (30 a, 30 b, . . . 30 n) includes a port (30 ai, 30 bi, . .. , 30 ni) coupled to the output port (28 ao 1, 28 bo 1, . . . , 28 no1) of one of the power factor correction units (28 a, 28 b, . . . , 28n), and also includes an output port (30 ao, 30 bo, . . . , 30 no) incommon with all output ports of the current sharing controllers (30 a,30 b, . . . 30 n). The current sharing controllers (30 a, 30 b, . . . ,30 n) are subject to surge current when the direct voltage of theassociated one of the power factor correction units (28 a, 28 b, . . . ,28 n) is coupled to the capacitive component (16) of the load (12) atturn-on. The apparatus (500, 600) further includes controlled currentpath (60) coupled to the source (20) of pulsating direct voltage (22)and to the load (12), for precharging the capacitive component (60) ofthe load (12).

In a more preferred embodiment, the apparatus (500) further comprises aplurality, equal to the first plurality (n), of ground currentequalizing impedances (R1, R2, . . . , Rn) coupled between a commonreference terminal (LG) and a current return port (28 ao 2, 28 bo 2, . .. , 28 no 2) of each of the power factor correction units (28 a, 28 b, .. . , 28 n). In a more preferred embodiment, the apparatus (600) furthercomprises means, such as a saturable reactor (50) for delayingapplication of the direct voltage to the load (12) by way of the currentsharing controllers (30 a, 30 b, . . . 30 n). In a most preferredembodiment, the current equalizing impedances are unidirectional currentconducting devices (210 a, 210 b, . . . , 210 n).

According to another aspect of the invention, the controllable path (60)includes a controllable switch. In a desirable embodiment of theinvention, the controllable switch includes a unidirectional currentconducting device which conducts when the pulsating direct voltage (22)is greater than the voltage on the capacitive component and which ceasesconduction when the pulsating direct voltage is less than the voltage onthe capacitive component. According to another aspect of the invention,the power factor correction units (28 a, 28 b, . . . , 28 n) are boostconverters which generate direct voltage having a value greater than thepeak value of the pulsating direct voltage, whereby the unidirectionalcurrent conducting means becomes nonconductive when the current sharingcontroller becomes conductive.

1. An electrical apparatus for powering a load including a resistive anda capacitive component, said electrical apparatus comprising: a sourceof pulsating direct voltage: a first plurality of power factorcorrection units coupled to said source of direct voltage for convertingsaid pulsating direct voltage into a direct voltage at an output portand for tending to maintain the current through said source of pulsatingdirect voltage in-phase with said pulsating direct voltage; a pluralityequal to said first plurality of current sharing controllers, each ofwhich includes a port coupled to said output port of one of said powerfactor correction units, and also includes an output port in common withall output ports of said current sharing controllers, said currentsharing controllers being subject to surge current when said directvoltage of the associated one of said power factor correction units iscoupled to said capacitive component of said load at turn-on; andcontrollable switch means coupled to said source of pulsating directvoltage and to said load, for precharging said capacitive component ofsaid load.
 2. An apparatus according to claim 1, wherein said powerfactor correction units are boost power factor correction converters. 3.An apparatus according to claim 1, further comprising a plurality, equalto said first plurality, of ground current equalizing impedances coupledbetween a common reference terminal and a current return port of each ofsaid power factor correction units.
 4. An apparatus according to claim1, further comprising means for delaying application of said directvoltage to said load.
 5. An apparatus according to claim 1, furthercomprising means for delaying application of said direct voltage to saidload by way of said current sharing controllers.
 6. An apparatusaccording to claim 1, wherein said controllable switch comprisesunidirectional current conducting means.
 7. An apparatus according toclaim 6, wherein said unidirectional current conducting means is poledto conduct when said pulsating direct voltage exceeds the load voltage.8. An apparatus according to claim 6, wherein said power factorcorrection units are boost converters which generate direct voltagehaving a first value greater than the peak value of said pulsatingdirect voltage, whereby said unidirectional current conducting meansbecomes nonconductive when said pulsating direct voltage drops belowsaid first value.
 9. An apparatus according to claim 1, wherein saidcontrollable switch includes a unidirectional current conducting devicewhich conducts when said pulsating direct voltage is greater than thevoltage on said capacitive component and which ceases conduction whensaid pulsating direct voltage is less than said voltage on saidcapacitive component.
 10. An apparatus according to claim 1, whereinsaid power factor correction units are voltage-boosting units whichproduce a direct voltage greater than the peak value of said pulsatingdirect voltage.
 11. An electrical apparatus for powering a loadincluding a resistive and a capacitive component, said electricalapparatus comprising: a source of pulsating direct voltage: a firstplurality of boost power factor correction units coupled to said sourceof direct voltage for converting said pulsating direct voltage into adirect voltage at an output port and for tending to maintain the currentthrough said source of pulsating direct voltage in-phase with saidpulsating direct voltage; a plurality equal to said first plurality ofcurrent sharing controllers, each of which includes a port coupled tosaid output port of one of said power factor correction units, and alsoincludes an output port in common with all output ports of said currentsharing controllers, said current sharing controllers being subject to asurge current when said direct voltage of the associated one of saidpower factor correction units is coupled to said capacitive component ofsaid load at turn-on; and unidirectional current conducting meanscoupled to said source of pulsating direct voltage and to said load,poled for charging said capacitive component of said load when saidpulsating direct voltage exceeds the load voltage.